Polymer composite strengthened with carbon fiber surface-modified by plasma treatment and method for producing polymer composite

ABSTRACT

Provided are an engineering plastic composite and a method for producing the same. The engineering plastic composite includes a carbon fiber having a surface modified by a hydrogen plasma and including a functional group and an engineering plastic. The carbon fiber is mixed with the engineering plastic to constitute a composite.

TECHNICAL FIELD

The present disclosure relates to carbon fiber reinforced polymercomposites and, more particularly, to a carbon fiber reinforced polymercomposite with a surface modified carbon fiber through plasma treatment.

BACKGROUND

Engineering plastics are superior in strength, elasticity, and thermalstability as well as chemical resistance and electrical insulation, ascompared to general-purpose plastics. Accordingly, the engineeringplastics have recently been applied to various industrial fields such ashousehold goods, electrical and electronic products, and aircraftstructural materials and various studies are being actively conducted onthe engineering plastics as candidate materials that can replace metals.

Engineering plastics may be classified into general engineering plasticsincluding five general-purpose engineering plastics with high usage andhigh-performance engineering plastics with excellent heat resistance.However, these high-performance engineering plastic materials have ahigher production cost than other plastics in spite of their excellentperformance. Thus, unlike general-purpose plastics which are easy inmass produce, the high-performance plastic materials are mainly focusedon customized development and production required for end use.

As a result, actual applications of high-performance plastic materialshave been quite limited up to now due to their relatively lowerhigh-temperature mechanical properties than metal materials inautomobile parts and turbine materials for metal replacement which maybe most demanding.

Carbon fibers are low-cost reinforced materials which are light andstrong, have a high modulus of elasticity, and are also widely used ingeneral-purpose plastics including engineering plastics.

In general, commercially available plastics including engineeringplastics are provided as carbon fiber/plastic composites with carbonfiber added. Strength improvement of several times to several tens oftimes may be achieved depending on the amount of the carbon fiber added.Carbon fibers may be used by mixing various types of carbon fibers suchas PAN-based carbon fibers, Pitch-based carbon fibers, and Rayon-basedcarbon fibers according to the types of plastic such as thermosettingand thermoplastic.

Each carbon fiber is subjected to a sizing treatment to coat the carbonfiber with an interfacial binder to achieve a stable physical interfacebetween plastic resins. When a composite material is prepared by mixingthe sizing-treated carbon fiber with a plastic, the composite materialexhibits very excellent reinforcing effect at room temperature butexhibits very low interfacial stability between a plastic base materialand a surface of the carbon fiber at high temperature. For this reason,high-temperature reinforcing effect is negligible, which is not suitableas a reinforcing method for high heat-resistance engineering plastics.

To overcome these disadvantages, applying a solvent solution of a sizingagent containing a polyglycidyl ether to a carbon fiber as a method formodifying a carbon fiber surface is disclosed (Patent Document 1).However, the method is not preferable due to probability ofenvironmental contamination caused by addition of acid or chemical.

In addition, a method for modifying a surface of a carbon fiber usingatmospheric pressure plasma during the production of the carbon fiber isdisclosed (Patent Document 2). However, the method is limited in usingexcellent characteristics of a pure carbon fiber because a nano-thinfilm is formed on a surface of the carbon fiber.

A technology proposed in the present disclosure to overcome theabove-described disadvantages reduces the manufacturing cost through anenvironment-friendly and simple process by plasma-treating acommercially available carbon fiber and induces mechanical and chemicalbonding between a plastic and a carbon fiber by modifying a surface ofthe carbon fiber and providing a functional group to improve mechanicalcharacteristics. Accordingly, the technology is aimed at securing stablemechanical properties even at high temperature to widen its use range.

Patent Document 1: Japanese Patent Publication No. 50-59589

Patent Document 2: Korean Patent Publication No. 10-2012-0055042

SUMMARY

The present disclosure relates to a carbon fiber reinforced engineeringplastic composite and a method for producing the same which adjusts achemical function group on a carbon fiber surface through a plasmatreatment of a conventional carbon fiber and thus promote bonding forcewith a plastic base material at high temperature to improve mechanicalcharacteristics at room temperature and high temperature and wear andfriction characteristics.

Example embodiments of the present disclosure provide an engineeringplastic composite. The engineering plastic composite includes a carbonfiber having a surface modified by a hydrogen plasma and including afunctional group and an engineering plastic. The carbon fiber is mixedwith the engineering plastic to constitute a composite.

In example embodiments, the modified carbon fiber may include functionalgroups containing carbon (C) and hydrogen (H).

In example embodiments, the modified carbon fiber may be in the rangefrom 20 to 30 percent by volume of the engineering plastic.

In example embodiments, the carbon fiber may be any one of a PAN-basedcarbon fiber, a PITCH-based carbon fiber, and a Rayon-based carbonfiber. A surface of the carbon fiber may be coated with polyurethane,and the polyurethane may be removed by a hydrogen plasma treatment.

In example embodiments, a tensile strength of the engineering plasticmay be less than or equal to 239 MPa at room temperature. A tensilestrength of the engineering plastic composite may be less than or equalto 150 MPa at temperature of 150 degrees Celsius.

In example embodiments, a coefficient of friction of the engineeringplastic composite may be less than or equal to 0.12.

In example embodiments, a yield strength of the engineering plasticcomposite may be less than or equal to 149 MPa.

In example embodiments, a modulus of elasticity of the engineeringplastic composite may be less than or equal to 25 GPa.

In example embodiments, a strength of the engineering plastic compositemay be less than or equal to 35.7 MN/m.

In example embodiments, wear amount of the engineering plastic compositemay be less than or equal to 1.17×10⁻⁹ mm³/N·m.

Example embodiments of the present disclosure provide a method forproducing an engineering plastic composite. The method includesperforming a plasma treatment using a reactive gas including a carbonfiber bundle to modify a surface of the carbon fiber bundle and attach afunctional group and mixing the modified carbon fiber with anengineering plastic to constitute a composite.

In example embodiments, the reactive gas may be a hydrogen gas (H₂).

In example embodiments, a surface of the carbon fiber bundle may becoated with polyurethane, and the polyurethane may be removed by theplasma treatment.

In example embodiments, the plasma treatment may be performed in aplasma apparatus which is disposed inside a vacuum container andincludes a top electrode and a bottom electrode disposed to face eachother by a capacitively-coupled hydrogen plasma which is generated by RFpower which places the carbon fiber bundle at the bottom electrode,heats the bottom electrode, and receives and provides a hydrogen gas tothe top electrode.

In example embodiments, a process pressure of the vacuum container maybe between 50 and 1000 milliTorr (mTorr), a reaction temperature of thebottom electrode may be between 300 and 700 degrees Celsius, and powerapplied to the top electrode may be between 100 and 1000 watts (W).

In example embodiments, the carbon fiber may be in the range from 20 to30 percent by volume of the engineering plastic.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more apparent in view of the attacheddrawings and accompanying detailed description. The embodiments depictedtherein are provided by way of example, not by way of limitation,wherein like reference numerals refer to the same or similar elements.The drawings are not necessarily to scale, emphasis instead being placedupon illustrating aspects of the present disclosure.

FIG. 1 is a conceptual diagram of a plasma apparatus for plasma-treatinga carbon fiber according to an example embodiment of the presentdisclosure.

FIG. 2 shows an FT-IR result indicating an infrared absorption spectrumof a hydrogen plasma-treated carbon fiber according to an exampleembodiment of the present disclosure.

FIG. 3 shows a Raman spectroscopy result of a hydrogen plasma-treatedcarbon fiber according to an example embodiment of the presentdisclosure.

FIG. 4 shows a result indicating an integrated signal ratio of a D peakand a G peak of the Raman analysis result in FIG. 3.

FIG. 5 is a graph showing an average graphene size (La) and an averagecrystal size (Leq) obtained by analyzing the Raman analysis result inFIG. 3.

FIG. 6A shows an X-ray photoelectron spectroscopy (XPS) result of ahydrogen plasma-untreated carbon fiber.

FIG. 6B shows an X-ray photoelectron spectroscopy (XPS) result of ahydrogen plasma-treated carbon fiber.

FIG. 6C shows X-ray photoelectron spectroscopy (XPS) result analysis ofa hydrogen plasma-untreated carbon fiber and a hydrogen plasma-treatedcarbon fiber.

FIG. 7A shows a result obtained by measuring surface roughness beforeand after hydrogen plasma treatment.

FIG. 7B is an atomic-force microscopy (AFM) image to check surfaceroughness before and after hydrogen plasma treatment.

FIG. 8A is a graph of performing a tensile test of an engineeringplastic composite including a plasma-untreated carbon fiber according toan example embodiment of the present disclosure.

FIG. 8B is a graph of performing a tensile test of an engineeringplastic composite including a plasma-treated carbon fiber according toan example embodiment of the present disclosure.

FIG. 9 shows a tensile strength depending on a carbon fiber volume ratioof a composite including a hydrogen plasma-treated carbon fiber and ahydrogen plasma-untreated carbon fiber.

FIG. 10 shows a modulus of elasticity depending on a carbon fiber volumeratio of a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

FIG. 11 shows a yield strength depending on a carbon fiber volume ratioof a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

FIG. 12 shows a stiffness depending on a carbon fiber volume ratio of acomposite including a plasma-treated carbon fiber and a plasma-untreatedcarbon fiber.

FIG. 13 shows a friction coefficient depending on a carbon fiber volumeratio of a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

FIG. 14 shows a wear rate depending on a carbon fiber volume ratio of acomposite including a plasma-treated carbon fiber and a plasma-untreatedcarbon fiber.

FIG. 15 shows a scanning electron microscope (SEM) image of a compositeincluding a plasma-treated carbon fiber and a composite including aplasma-untreated carbon fiber.

FIG. 16 is a graph showing tensile strengths depending on a measuredtemperature and a component and a measured temperature of a composite.

DETAILED DESCRIPTION

According to example embodiments of the present disclosure, a carbonfiber surface plasma treatment is performed to modify its surface,attach a chemical functional group, adjust a bonding structure ofcarbon, and adjust roughness. The modified carbon fiber is mixed with anengineering plastic to constitute a composite. The composite providesroom-temperature and high-temperature mechanical characteristics andhigh wear and friction characteristics. The composite is an engineeringplastic composite for high heat resistance, wear resistance, and lowfriction and includes a hydrogen plasma-treated carbon fiber as ahigh-temperature reinforcement.

According to example embodiments of the present disclosure, a hydrogenplasma treatment is performed to produce chemical functional groups suchas C—C, C═C, —CH, C—H, and OH on a carbon fiber surface. The carbonfiber surface is modified by hydrogen plasma to transform carbon bonds,i.e., transform an SP² bond of graphite structure to an SP³ bond ofdiamond structure. The carbon fiber may be any one of a PAN-based carbonfiber, a Pitch-based carbon fiber, and a Rayon-based carbon fiber.

Example embodiments will now be described more fully with reference tothe accompanying drawings, in which some example embodiments are shown.Example embodiments may, however, be embodied in many different formsand should not be construed as being limited to the embodiments setforth herein; rather, these example embodiments are provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of example embodiments of the present disclosure to those ofordinary skill in the art. In the drawings, the thicknesses of layersand regions are exaggerated for clarity. Like reference charactersand/or numerals in the drawings denote like elements, and thus theirdescription may be omitted.

FIG. 1 is a conceptual diagram of a plasma treatment apparatus 100 forplasma-treating a carbon fiber according to an example embodiment of thepresent disclosure.

Referring to FIG. 1, the plasma treatment apparatus 100 includes avacuum chamber 112 and a top electrode 114 and a bottom electrode 116provided inside the vacuum chamber 112 and disposed to face each other.The top electrode 114 is supplied with radio-frequency (RF) power froman RF power supply 132 through a matching network 134. The top electrode114 may generate a capacitively-coupled plasma. The top electrode 114may receive a reactive gas from an external entity and distribute andemit the gas. The reactive gas is a hydrogen gas. A frequency of the RFpower may be 13.56 MHz, and the power supplied to the top electrode 114may be several tens of watts (W) to several kilowatts (kW).

In a process of plasma-treating a commercially available carbon fiberbundle, besides a reactive gas, an inert gas such as argon may be addedto adjust discharge characteristics of the gas or form a functionalgroup.

The bottom electrode 116 may include a heater 118 therein and may begrounded. A ceramic crucible 119 is disposed on the bottom electrode116. The crucible 119 stores a commercially available carbon fiberbundle 10. The crucible 119 is heated by the heater 118. A temperatureof the heater 118 or the crucible 119 may be between 300 and 700 degreesCelsius. When the temperature of the crucible 119 is too high, a carbonfiber may be deposited by a contaminant. Preferably, the temperature ofthe crucible 119 may be between 300 and 400 degrees Celsius. Thereactive gas is supplied after the temperature of the crucible 119reaches a predetermined process temperature. When a pressure isstabilized, RF power is supplied to the top electrode 114 to generate ahydrogen plasma.

Preferable, the reaction temperature is 300 degrees Celsius. Since asufficient reaction cannot be performed when the reaction temperature isless than 200 degrees Celsius, the reaction temperature is notpreferable. When the reaction temperature is higher than 1000 degreesCelsius, a carbon fiber may be physically or chemically changed by theextremely high temperature. Hence, the reaction temperature higher than1000 degrees Celsius is not preferable.

A plasma treatment time of the commercially available carbon fiber maybe between several minutes and several hours. The commercially availablecarbon fiber has a length of 10 millimeters (mm) and a thickness of 5micrometers (μm), and a surface of the commercially available carbonfiber is coated with polyurethane.

A process pressure of the vacuum chamber 112 may be in the range of 100to 100 milliTorr (mTorr) and may be preferably 250 mTorr.

A hydrogen plasma treatment time of the carbon fiber may be between oneand five hours. The hydrogen plasma causes a chemical reaction toactivate (or etch) a substrate surface while modifying a carbon crystalstructure and producing chemical functional groups on the surface.

A method for producing a carbon fiber reinforced engineering plasticcomposite according to an example embodiment of the present disclosureincludes plasma-treating a carbon fiber bundle with ahydrogen-containing reactive gas to modify a surface of the carbon fiberbundle and attach a functional group and mixing the modified carbonfiber with an engineering plastic to constitute a composite.

The engineering plastic includes one or more high-functional engineeringplastics selected from the group consisting of polyphenylene sulfide(PPS), polyether imide (PEI), polyether sulfone (PES), polyarylate(PAR), polyether ether ketone (PEEK), and tetrafluoroethylene resin(PTFE).

The carbon fiber content of the composite may be 20 to 30 percent byvolume of the engineering plastic. When the carbon fiber content is outof the above range, mechanical characteristics includinghigh-temperature strength and friction characteristics are deteriorated.Therefore, the carbon fiber content out of the above range is notpreferable.

The carbon fiber reinforced engineering plastic composite may have anysize and shape according to a plastic molding method.

FIG. 2 shows an FT-IR result indicating an infrared absorption spectrumof a hydrogen plasma-treated carbon fiber according to an exampleembodiment of the present disclosure.

Referring to FIG. 2, the hydrogen plasma-treated carbon fiber employed acommercially available carbon fiber as a starting material. Thecommercially available carbon fiber has a surface coated withpolyurethane and has a length of 10 mm and a thickness of 5 μm. Thecarbon fiber was treated for an hour in a plasma treatment apparatus 100at RF power of 450 W, pressure of 250 mTorr, hydrogen flow rate of 100standard cubic centimeters per minute (sccm), and reaction temperatureof 300 degrees Celsius. Example 1 is an FT-IR analysis result of aplasma-treated sample, and it was confirmed that functional groups eachincluding carbon (C) and hydrogen (H) were produced.

Example 1 is a result obtained by measuring a plasma-untreatedcommercially available carbon fiber.

FIG. 3 shows a Raman spectroscopy result of a hydrogen plasma-treatedcarbon fiber according to an example embodiment of the presentdisclosure.

Referring to FIG. 3, Raman shift has a disordered peak (D peak) and agraphite peak (G peak). The D peak indicates an SP³ bond degree, and theG peak indicates an SP² bond degree.

FIG. 4 shows a result indicating an integrated signal ratio of a D peakand a G peak of the Raman analysis result in FIG. 3.

Referring to FIG. 4, a D peak integral signal I_(D) is a result ofintegrating an area of a D peak and a G peak integral signal I_(G) is aresult of integrating an area of a G peak. A signal ratio (I_(D)/I_(G))increases as a flow rate of hydrogen gas generating a plasma increases.Accordingly, the SP³ bond degree is increased by a hydrogen plasmatreatment of a carbon fiber to cause surface modification. ComparativeExample 1 is a hydrogen plasma-untreated result, and ExemplaryEmbodiment 1 is a plasma-treated result with a hydrogen gas flow rate of50 sccm.

FIG. 5 is a graph showing an average graphene size (La) and an averagecrystal size (Leq) obtained by analyzing the Raman analysis result inFIG. 3.

Referring to FIG. 5, both the average graphene size (La) and the averagecrystal size (Leq) degrease as a hydrogen plasma treatment is performed,which indicates that a surface of a carbon fiber is modified.

FIG. 6A shows an X-ray photoelectron spectroscopy (XPS) result of ahydrogen plasma-untreated carbon fiber.

Referring to FIG. 6A, a red dotted line (or black) is a result of ahydrogen plasma-untreated carbon fiber. The XPS spectrum may be dividedinto bond components. Blue indicates SP² bond, pink indicates SP³ bond,green indicates OH bond, and dark yellow indicates COOH bond.

FIG. 6B shows an X-ray photoelectron spectroscopy (XPS) result of ahydrogen plasma-treated carbon fiber.

Referring to FIG. 6B, a red dotted line (or black) indicates a result ofa plasma-untreated carbon fiber. The XPS spectrum may be divided intobond components. Blue indicates SP² bond, pink indicates SP³ bond, greenindicates OH bond, and dark yellow indicates COOH bond.

FIG. 6C shows X-ray photoelectron spectroscopy (XPS) result analysis ofa hydrogen plasma-untreated carbon fiber and a hydrogen plasma-treatedcarbon fiber.

Referring to FIG. 6C, the SP² bond is decreased by a hydrogen plasmatreatment and the SP³ bond, the OH bond, and the COOH bond are increasedby a plasma treatment. Thus, the carbon fiber is modified by thehydrogen plasma treatment and has a functional group.

FIG. 7A shows a result obtained by measuring surface roughness beforeand after hydrogen plasma treatment.

Referring to FIG. 7A, surface roughness Ra is increased by a hydrogenplasma treatment.

FIG. 7B is an atomic-force microscopy (AFM) image to check surfaceroughness before and after hydrogen plasma treatment.

Referring to FIG. 7B, surface roughness increases when a hydrogen plasmatreatment is performed

FIG. 8A is a graph of performing a tensile test of an engineeringplastic composite including a plasma-untreated carbon fiber according toan example embodiment of the present disclosure.

Referring to FIG. 8A, a composite was produced by mixing a hydrogenplasma-untreated carbon fiber with a commercially available polyetherimide (PEI) that is a high-strength engineering plastic. Samples wereprepared, where a volume ratio of the carbon fiber increased to zero,10, 20, 30, and 40 percent as compared to the volume of thehigh-strength engineering plastic, respectively.

FIG. 8B is a graph of performing a tensile test of an engineeringplastic composite including a plasma-treated carbon fiber according toan example embodiment of the present disclosure.

Referring to FIG. 8B, a composite was produced by mixing a hydrogenplasma-treated carbon fiber with a commercially available PEI that is ahigh-strength engineering plastic. Samples were prepared, where a volumeratio of the carbon fiber increased to zero, 10, 20, 30, and 40 percent,respectively. A composite including a PEI base material depending on avolume ratio was mixed by a kneader apparatus at temperature of 260degrees Celsius for 30 minutes to complete a composite in which a carbonfiber is uniformly distributed. The obtained composite material wascompressed/molded by applying a pressure of 35 MPa at temperature of 290degrees Celsius using a compression/molding apparatus to prepare asample having a thickness of 3.0 mm. In addition, a PEI sample wasprepared in the same manner as described above and functions of the PEIsample were compared.

FIG. 9 shows a tensile strength depending on a carbon fiber volume ratioof a composite including a hydrogen plasma-treated carbon fiber and ahydrogen plasma-untreated carbon fiber.

Referring to FIG. 9, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a higher tensile strength than acomposite including a hydrogen plasma-untreated carbon fiber.Accordingly, the composite including a hydrogen plasma-treated carbonfiber exhibits superior mechanical characteristics.

In the case of PEI to which a carbon fiber is not added at all, atensile strength is 104 MPa. On the other hand, in the case ofComparative Example 2 in which a plasma-untreated carbon fiber is added,a tensile strength significantly increases to 170 MPa in a sample of acarbon fiber volume ratio of 30. A sample of Exemplary Embodiment 2prepared from the plasma-treated carbon fiber exhibited a higher tensilestrength of 239 MPa than the sample prepared from Comparative Example 2at a carbon fiber volume ratio of 30.

Similarly, an elastic modulus, a yield strength, and stiffness exhibitedthe highest performance in a sample of a carbon fiber volume ratio of30. The plasma-treated carbon fiber, i.e., Exemplary Embodiment 2exhibited higher performance than Comparative Example 2. This is becausean bonding force with a base material is improved in the composite byproduction of a chemical functional group on a carbon fiber surface fora plasma carbon fiber treatment, change in structure and size of thecarbon fiber, and increase in roughness. In addition, ExemplaryEmbodiment 2 exhibited a higher room-temperature tensile strength than173 MPa that is the tensile strength of a cast iron used as acommercially available metal material.

FIG. 10 shows a modulus of elasticity depending on a carbon fiber volumeratio of a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

Referring to FIG. 10, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a greater modulus of elasticitythan a composite including a hydrogen plasma-untreated carbon fiber.Accordingly, the composite including a hydrogen plasma-treated carbonfiber exhibits superior mechanical characteristics. A modulus ofelasticity of a composite including a hydrogen plasma-treated carbonfiber having a volume ratio of 30 percent is 25 GPa.

FIG. 11 shows a yield strength depending on a carbon fiber volume ratioof a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

Referring to FIG. 11, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a higher yield strength than acomposite including a hydrogen plasma-untreated carbon fiber.Accordingly, the composite including a hydrogen plasma-treated carbonfiber exhibits superior mechanical characteristics. A yield strength ofa composite including a hydrogen plasma-treated carbon fiber having avolume ratio of 30 percent is 149 MPa.

FIG. 12 shows a stiffness depending on a carbon fiber volume ratio of acomposite including a plasma-treated carbon fiber and a plasma-untreatedcarbon fiber.

Referring to FIG. 12, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a higher stiffness than a compositeincluding a hydrogen plasma-untreated carbon fiber. Accordingly, thecomposite including a hydrogen plasma-treated carbon fiber exhibitssuperior mechanical characteristics. A stiffness of a compositeincluding a hydrogen plasma-treated carbon fiber having a volume ratioof 30 percent is 35.7 MN/m.

FIG. 13 shows a friction coefficient depending on a carbon fiber volumeratio of a composite including a plasma-treated carbon fiber and aplasma-untreated carbon fiber.

Referring to FIG. 13, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a lower friction coefficient than acomposite including a hydrogen plasma-untreated carbon fiber.Accordingly, the composite including a hydrogen plasma-treated carbonfiber exhibits superior mechanical characteristics. A frictioncoefficient of a composite including a hydrogen plasma-treated carbonfiber having a volume ratio of 30 percent is 0.12.

The lower a friction coefficient, the more preferable. It will beappreciated that a friction coefficient has a superior value when afriction coefficient of Exemplary Embodiment 2 of an engineering plasticwith a plasma-treated carbon fiber of 40 volume ratio added therein is0.12 and 0.43 in the case of PEI than when a friction coefficient ofComparative Example 2 of PEI with a carbon fiber of 40 volume ratioadded therein. This refers to the fact that as wear resistance isimproved by increased strength resulting from base reinforcement in anengineering plastic, a contact area between a material and a counterpartmaterial is reduced and thus a contact resistance is lowered to reduce afriction coefficient.

FIG. 14 shows a wear rate depending on a carbon fiber volume ratio of acomposite including a plasma-treated carbon fiber and a plasma-untreatedcarbon fiber.

Referring to FIG. 14, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a lower wear rate than a compositeincluding a hydrogen plasma-untreated carbon fiber. Accordingly, thecomposite including a hydrogen plasma-treated carbon fiber exhibitssuperior wear characteristics.

Comparative Example 2 is a PEI composite to which the plasma-untreatedcarbon fiber is added. Exemplary Embodiment 2 is a PEI composite towhich the plasma-treated carbon fiber is added. It will be appreciatedthat a wear rate is 1.17×10⁻⁹ mm³/N·m in the case of PEI to which aplasma-treated carbon fiber of 40 volume ratio is added, which issignificantly lower than 2.14×10⁻⁹ mm³/N·m that is a wear rate in thecase of a sample to which a typical carbon fiber of 40 volume ratio isadded. As described above, a friction coefficient is lowered by theeffect resulting from base reinforcement caused by chemical bonding toan engineering plastic when a carbon fiber is plasma-treated.

Exemplary Embodiment 2 (the composite to which a plasma-treated carbonfiber is added) exhibited better room-temperature and high-temperaturetensile strength, yield strength, modulus of elasticity, stiffness,coefficient of friction, and wear amount than Comparative Example 2 (theengineering plastic to which an untreated carbon fiber is added).

FIG. 15 shows a scanning electron microscope (SEM) image of a compositeincluding a plasma-treated carbon fiber and a composite including aplasma-untreated carbon fiber.

Referring to FIG. 15, a sample was prepared in the same manner asdescribed in FIGS. 8A and 8B. A composite including a hydrogenplasma-treated carbon fiber exhibits a bonding force with polyetherimide (PE), which is a base material, as compared to a compositeincluding a hydrogen plasma-untreated carbon fiber. It is confirmed thatmore PEIs, each being a base material, bond to a carbon fiber surface inthe composite produced from Exemplary Embodiment 2 than to a carbonfiber surface in the composite produced from Comparative Example 2. Thisis because a chemical functional group and a carbon crystal structureare transformed on a carbon fiber surface by a plasma treatment toimprove the bonding force with the base material in the composite.

FIG. 16 is a graph showing tensile strengths depending on a measuredtemperature and a component and a measured temperature of a composite.

Referring to FIG. 16, Comparative Example 1 includes only PEI andComparatively Example 2 is a composite in which a plasma-untreatedcarbon fiber having a volume ratio of 30 percent is mixed with PEI.Comparative Example 3 is cast iron which is a metal. ExemplaryEmbodiment 2 is a composite in which a plasma-treated carbon fiberhaving a volume ratio of 30 percent is mixed with PEI.

A tensile strength was measured at room temperature and temperature of150 degrees Celsius by using samples. As compared to the tensilestrength at the temperature of 150 degrees Celsius, a high-temperaturetensile strength of PEI and a tensile strength of Comparative Example 2to which a plasma-untreated carbon fiber is added exhibit ahigh-temperature tensile strength of 60 MPa which is nearly similarthereto. Accordingly, performance of a conventional carbon fiberreinforced engineering plastic composite is significantly deterioratedat the high temperature of 150 degrees Celsius.

However, in the sample of Exemplary Embodiment 2, a high-temperaturetensile strength rapidly increases due to the plasma treatment effect ofa carbon fiber. A value of 150 MPa is similar to a value of ComparativeExample 3. Thus, a composite according to an example embodiment of thepresent disclosure exhibits a higher tensile strength at roomtemperature than a metal material and also exhibits a similar tensilestrength even at high temperature of 150 degrees Celsius to the metalmaterial. As a result, the composite according to an example embodimentof the present disclosure may replace the metal material.

As described above, when a carbon fiber is treated with a plasma, achemical functional group and a carbon bond structure are adjusted on asurface of the carbon fiber. Thus, the surface-modified carbon fiber mayimprove a bonding force with a plastic base material and frictioncharacteristics at room temperature and high temperature. As a result, acarbon fiber reinforced polymer composite may provide high-strength,wear-resistance, and low-friction characteristics.

A high-strength engineering plastic including a carbon fiber has higherthermal characteristics than a conventional general-purpose plastic.However, when the high-strength engineering plastic including a carbonfiber is actually used in a mechanical component material for hightemperature, it is severely damaged or worn. When a plasma-treatedcarbon fiber according to example embodiments of the present disclosureis used, a bonding force with a base material is promoted in anengineering plastic due to a functional group prepared on a surface ofthe carbon fiber to improve a high-temperature strength and frictioncharacteristics.

Accordingly, a heat-resistance engineering plastic composite producedfrom the present disclosure may be used as a coating material of acomponent surface that requires heat-resistance characteristics.Especially, the heat-resistance engineering plastic composite may besuitably used as a material that replaces a mechanical metal materialsuch as an automobile component, a high-temperature turbine or the like.

Although the present disclosure and its advantages have been describedin detail. it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the following claims.

What is claimed is:
 1. An engineering plastic composite comprising: acarbon fiber having a surface modified by a hydrogen plasma andincluding a functional group; and an engineering plastic, wherein: thecarbon fiber is mixed with the engineering plastic to constitute acomposite.
 2. The engineering plastic composite as set forth in claim 1,wherein: the modified carbon fiber includes functional groups containingcarbon (C) and hydrogen (H).
 3. The engineering plastic composite as setforth in claim 1, wherein: the modified carbon fiber is in the rangefrom 20 to 30 percent by volume of the engineering plastic.
 4. Theengineering plastic composite as set forth in claim 1, wherein: thecarbon fiber is any one of a PAN-based carbon fiber, a PITCH-basedcarbon fiber, and a Rayon-based carbon fiber, a surface of the carbonfiber is coated with polyurethane, and the polyurethane is removed by ahydrogen plasma treatment.
 5. The engineering plastic composite as setforth in claim 1, wherein: a tensile strength of the engineering plasticis less than or equal to 239 MPa at room temperature, and a tensilestrength of the engineering plastic composite is less than or equal to150 MPa at temperature of 150 degrees Celsius.
 6. The engineeringplastic composite as set forth in claim 1, wherein: a coefficient offriction of the engineering plastic composite is less than or equal to0.12.
 7. The engineering plastic composite as set forth in claim 1,wherein: a yield strength of the engineering plastic composite is lessthan or equal to 149 MPa.
 8. The engineering plastic composite as setforth in claim 1, wherein: a modulus of elasticity of the engineeringplastic composite is less than or equal to 25 GPa.
 9. The engineeringplastic composite as set forth in claim 1, wherein: a strength of theengineering plastic composite is less than or equal to 35.7 MN/m. 10.The engineering plastic composite as set forth in claim 1, wherein: wearamount of the engineering plastic composite is less than or equal to1.17×10⁻⁹ mm³/N·m.
 11. A method for producing an engineering plasticcomposite, comprising: performing a plasma treatment using a reactivegas including a carbon fiber bundle to modify a surface of the carbonfiber bundle and attach a functional group; and mixing the modifiedcarbon fiber with an engineering plastic to constitute a composite. 12.The method as set forth in claim 11, wherein: the reactive gas is ahydrogen gas (H₂).
 13. The method as set forth in claim 11, wherein: asurface of the carbon fiber bundle is coated with polyurethane, and thepolyurethane is removed by the plasma treatment.
 14. The method as setforth in claim 11, wherein: the plasma treatment is performed in aplasma apparatus which is disposed inside a vacuum container andincludes a top electrode and a bottom electrode disposed to face eachother by a capacitively-coupled hydrogen plasma which is generated by RFpower which places the carbon fiber bundle at the bottom electrode,heats the bottom electrode, and receives and provides a hydrogen gas tothe top electrode.
 15. The method as set forth in claim 14, wherein: aprocess pressure of the vacuum container is between 50 and 1000milliTorr (mTorr), a reaction temperature of the bottom electrode isbetween 300 and 700 degrees Celsius, and power applied to the topelectrode is between 100 and 1000 watts (W).
 16. The method as set forthin claim 11, wherein: the carbon fiber is in the range from 20 to 30percent by volume of the engineering plastic.